Tube mill such as ball mill having a high pulverizing capability has heretofore been used for the pulverization of solid materials of a high hardness such as cement clinker and blast furnace slag. But, such tube mill is less efficient and increases the running cost and is therefore very uneconomical.
Under the circumstances, various studies have been made from the desire that the above roller mill which is relatively satisfactory in point of efficiency be utilized for the pulverization of cement clinker and blast furnace slag.
However in the case of roller mill, unlike tube mill, the pulverization of raw material is performed not by collision between pulverizing medium such as balls and raw material and by grinding, but raw material stuck between a pulverizing table and pulverizing rollers both supported by a machine frame is to be pulverized by virtue of the gripping force of both table and rollers. Consequently, vibrations of the pulverizing rollers, etc. are in many cases transferred to the machine frame, thereby creating extremely large vibrations as compared with the use of tube mill. For this reason, there are many who hesitate to use roller mill for the pulverization of very hard solid materials such as cement clinker and blast furnace slag.
Roller mill is generally considered superior in pulverization efficiency to tube mill, but the efficiency of roller mill presently available is not always satisfactory and it is considered that there is a considerable room for improvement.
Vibrations in such roller mill, above all, those induced by the vibration or pulverizing rollers, are broadly classified into (1) those attributable to the hardness of raw material or changes thereof and (2) so-called self-excited vibrations caused by slipping of raw material. The present invention aims at reducing the latter or self-excited vibration and improving pulverization efficiency. The cause of such self-excited vibration will be explained below with reference to FIGS. 1 to 5.
FIG. 1 is a sectional side view showing an example of structure of a conventional general roller mill, in which the reference numeral 1 denotes a pulverizing table which is positively rotated in a horizontal plane about a vertical axis 2 by means of a drive source such as a motor (not shown).
In the upper surface of the pulverizing table 1 is formed an annular groove 3 around the vertical shaft 2. As shown in the figure, the annular groove 3 has an arcuate section which is depressed downward.
Mounted above the pulverizing table 1 are a set of pulverizing rollers 5a and 5b whose outer periphreal surfaces 4 are opposed to and urged toward the annular groove 3 through a gap 6.
More specifically, the pulverizing rollers 5a and 5b are rotatably supported by roller shafts 9a and 9b which are inserted into a pulverizing chamber 8 through a casing 7. The roller shafts 9a and 9b are fixed to frames 11a and 11b which are swingably on horizontal shafts 10a and 10b in a vertical plane, the shafts 10a and 10b being provided outside the body casing 7. A bolt 13a (only one being shown) is threadedly secured to an arm 12a (only one being shown), and the head of the bolt 13a (13b ) is adapted to abut the frame 11a (11b), thereby setting the minimum limit of the width of the gap 6 between the pulverizing rollers 5a, 5b and the annular groove 3.
The fore end portions of the above set of frames 11a and 11b are connected so that they can be pulled by a tension device 14 and rods 15a and 15b. Consequently, the top portion sides of the frames 11a and 11b undergo pivotal urging forces in directions approaching each other as indicated with arrows A, whereby the pulverizing rollers 5a and 5b are urged toward the annular groove 3. In this case, the pivotal motion of the frames 11a and 11b in the direction of arrow A is restricted by the bolts 13a and a similar bolt (not shown) as mentioned above, and after the minimum limit width of the gap 6 is set.
Therefore, the raw material fed to the central part of the upper surface of the pulverizing table 1 is moved in the outer peripheral direction, that is, into the annular groove 3, by the truncated cone-like upper surface shape formed at the central part of the pulverizing table 1 and the centrifugal force created by the rotation of the pulverizing table 1, and is stuck into the gap 6 between the pulverizing rollers 5a, 5b and the pulverizing table 1 and thereby pulverized under pressure.
But, in the event the thickness of the raw material layer stuck below one pulverizing roller, e.g. 5a, is too large, the pulverizing roller 5a pivots in an upwardly escaping direction against the pivotal urging force of the tension device 14, and its pivoting force is transmitted through the tension device 14 and rod 15b to the frame 11b to which is attached the opposite-side pulverizing roller 5b, so that the urging force of the pulverizing roller 5b toward the annular groove 3 is enhanced. Thus, the urging forces of the pulverizing rollers 5a and 5b are automatically adjusted according to changes in layer thickness of raw material.
In this way, the raw material which has been pulverized by the rollrs 5a and 5b moves to the outer peripheral side of the pulverizing table 1 by virtue of the centrifugal force of the pulverizing table 1, and is blown up by an upwardly air current introduced from an upward nozzle 16 which surrounds the outer periphery of the pulverizing table 1. Then, separation by particle size is performed by means of a separator (now shown) disposed at an upper part of the pulverizing chamber 8, and only fine particles not larger than a predetermined particle size are taken out of the pulverizing chamber 8, while coarse particles larger than the predetermined particle size are returned to the upper surface of the pulverizing table 1 and again subjected to pulverization treatment.
As shown in FIG. 2, in the conventional roller mill, between a radius of curvature, r, of the outer peripheral surface 4 of the pulverizing roller 5a (5b) when cut by a plane including the roller shaft 9a or 9b and a radius of curvature, R, of the annular groove 3 when cut by a plane including the roller shaft 9a or 9b, there exists the relationship of R&gt;r.
In the example shown in FIG. 2(a), R=R.sub.1, r=r.sub.1, R.sub.1 =r.sub.1 +d.sub.1, d.sub.1 =d.sub.0, and thus the thickness, d, in the roller radial direction of the gap 6 between both curved surfaces is constant (d.sub.1 =d.sub.0). In the example shown in (b) of the same figure, R=R.sub.1, r=r.sub.2, R.sub.1 &gt;r.sub.2 +d.sub.2, d.sub.2 &gt;d.sub.0, and the thickness, d, of the gap 6 between both curved surfaces is set so that the thickness, d.sub.2, of the front or rear end side is always larger than the thickness, d.sub.0, of the central part.
In the case of roller mill, moreover, as shown in FIG. 3 which is a front view of the pulverizing roller 5, the pulverization of raw material is performed not at a point 16 just under the roller at which the compression is maximum, but at a point 17 (located behind by distance from the roller center) located this side (right-hand side in the figure) when viewed in the advancing direction (arrow) of the pulverizing table 1. Further, as shown in FIG. 4 which is a plan view of the pulverizing roller 5, the peripheral speed F.sub.4 in the rotating direction of the outer periphery of the roller 5 shifts by an angle of .alpha. relative to a peripheral speed F.sub.3 in the rotating (tangential) direction of the pulverizing table 1 at the sticking point 17, and in accordance with this shift angle a shearing force in the direction of F.sub.5 acts on the raw material located just under the sticking point 17. Flow of the raw material powder is induced also by this shearing force F.sub.5, which is presumed to enhance the self-excited vibration.
Further, comparison between the peripheral speed of the outer peripheral surface 4 of the pulverizing roller 5 in the vicinity of the sticking point 7 and that of the annular groove 3 of the pulverizing table 1 can be diagrammatically shown as in FIG. 5. More specifically, the peripheral speed on the side of the annular groove 3 is proportional to the radius from the rotational center 0 of the pulverizing table 1. For example, when the outer peripheral surface 4 of the pulverizing roller 5 is viewed in the width direction, if the peripheral speed at point 17a relatively close to the center 0 is Va and that at point 17b relatively far from the center 0 is Vb, there exists the relation of Vb&gt;Va, and since the peripheral speed Vo of the peripheral surface 4 of the pulverizing roller 5 is a mean value of Va and Vb, there exists the relation of Vb&gt;Vo&gt;Va.
Between the outer peripheral surface 4 of the pulverizing roller 5 and the annular groove 3 there occurs a slip which is caused by the above difference in peripheral speed, and a shearing force induced by this slip creates flow of the raw material powder in the gap 6, which is cause of occurrence of the foregoing self-excited vibration.
Thus, the self-excited vibration of the pulverizing roller 5 is caused by the flow of the raw material powder in the direction of the roller shaft 9 in the gap 6. In this connection, in the conventional roller mill, as shown in FIGS. 1 and 2, the thickness of the gap 6 is either constant when viewed in the direction of the roller shaft 9 [FIG. 2(a)] or is larger at the front or rear end portion than at the central portion [FIG. 2(b)]. In any event anyhow, the gap 6 is in a forwardly or rearwardly opened state, not assuming a shape capable of preventing the flow of raw material formed in the gap 6, and thus the structure permits easy occurrence of self-excited vibrations.
Further, the pulverization in the roller mill is effected by both compressive and shearing forces. If the respective force regions are called adhesion region (compression region) A and slip region (shear region) B, as shown in FIG. 6, the raw material which has been coarsely pulverized in the adhesion region is finely pulverized in the slip region. At this time, is a force is applied to a powder layer C by the roller, it is very likely with a roller shape as in FIG. 2(a) that the powder layer C will flow out to the right and left sides of the roller. Moreover, the larger the width or diameter of the roller, the larger the slip region B, and in this case the shearing force is enhanced, so that the efflux probability of paticles becomes larger. Both these influences combine to cause the powder efflux phenomenon. In short, the large area of the slip region B is the main cause and this combines with the defective roller shape to create said phenomenon.